1. Field of the Invention
The present invention relates to a multibeam image forming apparatus. More particularly, the invention relates to realization of high resolution of an image formed by the apparatus.
2. Description of the Related Art
In image forming apparatuses for forming an electrostatic latent image using a laser beam, such as digital copiers, laser-beam printers, and the like, a laser beam from a semiconductor laser is projected onto a rotating polygonal mirror, and a reflected beam from the polygonal mirror is projected onto the surface of a charged photosensitive member moving at a constant speed. By the rotation of the polygonal mirror, the laser beam is caused to perform scanning in a direction (main scanning direction) orthogonal to the moving direction (sub-scanning direction) of the photosensitive member. Since the laser beam is modulated in accordance with an image to be output, an electrostatic latent image is formed on the photosensitive member. The electrostatic latent image is developed to provide a visualized toner image.
Recently, in response to a request for high-speed image formation, multibeam image forming apparatuses having a plurality of lasers have been realized.
In such a multibeam image forming apparatus, lasers are disposed so as to perform scanning by arranging a plurality of laser beams in a sub-scanning direction. For example, a forced-laser-emission signal and a sample-and-hold request signal are prepared for each laser, and start of control to cause each laser to emit light by the forced-laser-emission signal and the sample-and-hold request signal at a predetermined timing, and to adjust the amount of laser light to a predetermined value (automatic power control) is instructed in a non-image region for each scanning operation during a printing operation. An image for a plurality of lines is formed at one laser scanning operation. Accordingly, in a printer of this type, an image for the same number of lines as the number of laser beams can be formed at one laser scanning operation. As a result, high-speed printing can be realized.
On the other hand, in order to respond to a request for high-resolution image formation, the number of image forming apparatuses having a high-resolution mode in addition to an ordinary-resolution mode is increasing (refer to Patent Literatures 1, 2).
When intending to provide high resolution in a main scanning direction, an approach to increase the frequency of video clock signals for transmitting image data is adopted in order to increase the pixel density at one laser scanning operation.
When intending to provide high resolution in a sub-scanning direction, an approach to increase the rotational speed of the polygonal mirror as well as increasing the frequency of video clock signals, or an approach to reduce the moving speed of the photosensitive member is adopted in order to increase the scanning frequency (the number of scanning operations per unit length in the sub-scanning direction) for the photosensitive member.
However, in a multibeam image forming apparatus, if the scanning frequency for the photosensitive member is increased in order to provide high resolution in the sub-scanning direction, a problem of duplicated scanning arises. For example, in a two-beam image forming apparatus, the scanning frequency for the photosensitive member must be increased by four times in order to double the resolution in the sub-scanning direction. In this case, if the two-beam image forming system is used without being modified, the same scanning line is subjected to duplicated scanning by respective laser beams because the beam interval remains in the state of original resolution (refer to FIG. 4 and related description to be provided below). Accordingly, scanning must be performed only by one of two beams, and therefore the feature of the two-beam image forming system is not utilized.
Furthermore, in order to provide high resolution in the main scanning direction, the frequency of video clock signals may be increased. However, an increase in the frequency causes a problem of radiation noise from circuitry, thereby increasing the production cost in order to deal with radiation noise.